Squatter

mouse (mous)
n., pl. mice (mīs).

Any of numerous small rodents of the families Muridae and Cricetidae, such as the common house mouse (Mus musculus), characteristically having a pointed snout, small rounded ears, and a long naked or almost hairless tail. 

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Found a little baby-looking one in the house this evening. Thing is, I swear I heard the cats having fun last night with some plaything I couldn't identify so I'm pretty sure it was it. That means she [it] was in the house all day, with Lily running around and probably forcing her to try on her clothes, hats and shoes.